1. Field of the Invention.
The present invention relates to the field of communication networks.
2. Prior Art.
In many current applications it is desired to have a plurality of devices connected in some form of network to allow digital communication to and from the various digital devices so connected. In general in such networks communication is in digital form, with the messages sent and received comprising various types of information depending upon the nature of the network and of the devices coupled to the network. By way of example, the information being communicated may be data, commands to control the actions of various devices, monitoring signals, acknowledgement signals, etc. For this purpose various types of basic networks are known, though each type in general has many variations, either enhancing the basic network or tailoring the network to specific applications or both. By way of example, some networks allow any device coupled to the network to seize control of the network under certain conditions, during which time the seizing device can communicate with any other device on the network. Such a scheme has the advantage of allowing device to device communications, though has certain complication in hardware to resolve network contention problems, determine priority, provide for expansion capabilities, etc. Other networks frequently use some form of central controller to control the network whereby the individual devices or nodes of the network may be of a simpler configuration because of the lack of need therein for the ability to seize and control the network. It is this general type of network to which the present invention is primarily directed.
In addition to the foregoing, there are various techniques which may be utilized to handle communication from the controller to the nodes and from the nodes back to the controller. By way of example, in a multidrop system, each node monitors a line and recognizes that when a specific request is made of that node, it will respond in accordance with that request. The advantage of the multidrop connection of course is that communication to all nodes is very fast, as all nodes are monitoring the same line at the same time. However, such a system has certain limitations in that while a specific node may quickly respond to a request addressed thereto, severe bus contention and priority problems arise if the use of a generalized pole is contemplated, whereby more than one node need respond at substantially the same time. Accordingly, such systems, to remain relatively inexpensive, must utilize individualized polling of nodes to avoid these problems.
In the case of a daisy chain connection, each node in a daisy chain's string of nodes will monitor an incoming request and if not for that node, will pass the request on down to the next node. Such a system has the advantage that line contention problems are avoided and priorities are set by the order in which the nodes are coupled in the daisy chain. However, such systems tend to be slow for various reasons and accordingly, are not suitable for use as high speed networks. Finally, some networks use time slotting wherein different nodes may communicate in different time slots. Again, line contention problems are alleviated, though the assignment of time slots to nonresponding nodes becomes an inefficient use of the potential communication rate of the system.
Reissue Pat. No. 28958 discloses a communication loop system having a bidirectional transmission capability between terminals utilizing a loop controller. The system includes switching units between some nodes which allow isolation of a loop segment and the establishment of an operable folded loop for the remaining nonisolated loop portions of the system. The system features devices for enabling synchronous switchover operation during reconfiguration of the loop, which synchronous switchover insures that no data stream interruption or disturbance occurs for the units or terminals not attached to the disconnected loop segment. To achieve this however, each switching unit requires two delay means and a discriminator so that one of two data streams may be delayed for synchronization before full system switch be achieved.
In U.S. Pat. No. 3,716,834, a system for use with a data transmission network having a central station and a plurality of remote stations connected in series by a low grade transmission line such as a telephone pair is disclosed. The system is immune to common transmission line faults and is also capable of identifying a transmission line fault when it occurs, the nature of the fault and the approximate location of the fault in the network. In U.S. Pat. No. 3,601,806, digital signals are bidirectionally transmitted between a master station and a plurality of remote stations on a single wire. Each station is successively interrogated by means of clock pulses transmitted from the master station, data being transmitted from the master station to the remote station on certain predetermined digital bits established by the clock pulses, and data being transmitted from the remote station to the master station on certain other predetermined bits. When the predetermined bit count for each remote station has been completed, this station is deactivated and clock and data pulses are bypassed through the station in either direction to provide communication between the master station and another remote station. This interrogation and bidirectional transmission of data is repeated successively for each remote station until all in turn have been interrogated or polled.